The present invention relates generally to optical distance sensing arrangements, and more particularly to calibration of optical distance sensing arrangements using position sensitive detectors (PSD).
Conventional optical distance measuring systems which utilize a PSD element to measure distance between the optical system and the surface of an object, such as used in autofocus cameras and other range measuring equipment, measure distance by triangulation. More specifically, as shown in FIG. 1, a block diagram of a conventional optical distance measuring system 10 includes an LED transmitter 12, a transmitter lens 14, a receiver lens 16, and a PSD element 18 with associated amplifier circuit 20. In operation, the LED emits a distancing light beam which is reflected in all directions by the surface of an object 22. The light which is reflected through the principal point of the receiver lens 16 forms an angle .alpha. relative to the incident beam. The reflected light is focused by the receiving lens 16 to form a beam spot on an active area of the PSD element 18. The various physical parameters shown in FIG. 1 are defined as follows:
x--distance of the object from principal point of transmitter lens along beam of incident distancing light; PA1 L--length of the active area of the PSD element; PA1 I--current from the inside lead of the PSD element; PA1 O--current from the outside lead of the PSD element; PA1 y--distance from the inside edge of the PSD active area to the center of the spot of reflected/scattered light; PA1 s--perpendicular distance from the center of the incident distancing light beam to the principal point of the receiver lens; PA1 f--distance between the principal point of the receiver lens and the plane of the PSD as measured parallel to the incident distancing light beam; PA1 q--distance from a point where a line through the principal point of the receiver lens and parallel to the incident distancing light beam intersects the plane of the PSD element to the inside edge of the PSD active area; and PA1 z--distance from a point where a line through the principal point of the receiver lens and parallel to the incident distancing light beam intersects the plane of the PSD element to the center of the reflected light spot on the PSD element; where EQU y=LO/(I+O); (1) EQU z=q+y; and EQU cot .alpha.=x/s=f/z, EQU x=(f)(s)/z (2)
As evidenced from equation (2), the distance measurement x is directly affected by the interpositioning of the PSD element, the receiver lens and the transmitter. Thus, manufacturing and/or assembly variations in f, s, and z cause large variations and errors in the distance measurement. This is particularly true the farther away the object is relative to the distance measuring system.
Generally, known optical distance measuring systems attempt to accommodate for manufacturing/assembly variations by incorporating a mechanical adjustment mechanism into the system. In this manner, detection of incorrect distance measurements due to slight variations in positioning can be corrected by adjusting the physical position of a desired system component. However, mechanical adjustments are unsatisfactory because of the substantial expense added to both the overall system and the manufacturing/assembly process therefor. In addition to measurement errors arising from physical misalignments, differences in gains between the PSD I and O amps can create measurement errors due to improper O and I current readings when calculating y. Thus, known optical distance measurement systems have not provided a satisfactory arrangement which can effectively compensate for variations in circuit component parameters and physical parameters, thereby compromising distance measurement accuracy.